The present invention relates to phosphors and imaging devices such as projection-type cathode ray tubes and display panels using the phosphors. More particularly, the invention relates to green emitting phosphors that emit high-luminance light with less luminance degradation and are suitable for image display and imaging devices such as projection-type cathode ray tubes and image display panels using such phosphors.
In recent years, imaging devices using cathode ray tubes for displaying color images have made advances toward higher solution and luminance to meet the need of higher performance. The imaging devices discussed herein are those for imaging from image data through light emission in such a way that phosphors are excited by irradiation of electron beams or ultraviolet rays; namely, cathode ray tubes (especially, projection-type cathode ray tubes), display panels using low energy electron beams, such as a field-emitter display (FED), and plasma display panels (PDP). The imaging devices include a system for imaging from image data which incorporates a drive for driving any tube or panel mentioned above, image data processing circuitry, and the like.
Higher solution and luminance of these imaging devices have been achieved by reducing the diameter of an excitation spot of electron beams or the like, increasing the scan speed, and increasing the excitation intensity. However, this is accompanied by the following drawbacks: luminance saturation of phosphors used in the imaging device, luminance degradation, and significant afterimages due to afterglow, resulting in a decline in image quality as a problem. At the same time, better color reproduction quality is required. Therefore, phosphors must satisfy requirements of luminance saturation, degradation, and afterglow properties and color enhancement.
Using a projection-type cathode ray tubes (hereinafter referred to as a projection tube) typical of the imaging devices as an example, its problems will be explained below. For the cathode ray tube, its luminance intensity is controlled by regulating the current of excited electron beams. Thus, it is required that the luminance of phosphors linearly increases in proportion to the current. However, generally, as the excitation intensity becomes high, luminance saturation takes place, that is, the luminance runs off the linear. When an image is displayed with highly intense excitation, phosphor materials are damaged. Consequently, luminance decreases and color emission degrades during continued use of the projection tube.
The projection tube is a cathode ray tube for use in a projection-type display and projects an image generated by the cathode ray tube on a screen through the optics that enlarge the image area by several tens of times. Thus, excitation is performed by current of 10 to 100 times as much as a generally used direct-viewing cathode ray tube producing non-enlarged images.
Accordingly, requirements for phosphors for the projection tube are less luminance saturation especially when the tube carries a large quantity of current and less degradation when the tube carries a large quantity of current. Among the phosphors used for emission of three primary colors R (read), B (green), and B (blue), especially for green emitting phosphors which generate 70% of luminance in a white light, the above improvements of phosphor properties are important.
Varieties of materials have so far been used for green emitting phosphors for the cathode ray tube. For example, a phosphor composition expressed by chemical formula Y2SiO5:Tb is known. The feature of this phosphor composition is less luminance saturation when excited with high-density current and it has been generally used as a practical phosphor. However, such a problem with this phosphor composition has been posed that a decline in coloring and color emission efficiency occurs by irradiation of electron beams and luminance degrades as irradiation continues. Another problem thereof is insufficient luminance of light it generates.
To solve these problems, luminance improvement was attempted by replacing a part of the composition with Sc as disclosed in, for example, Japanese Examined Patent Publication No. Sho 61-21505 and Japanese Examined Patent Publication No. Hei 6-62939.
Furthermore, as disclosed in, for example, Japanese Laid-Open No. Hei 2-289679, improvement to luminance and suppressing luminance degradation were attempted by replacing a part of the composition with any one of the substances Gd, Tm, Sm, and Eu.
Furthermore, as disclosed in, for example, the above-mentioned Japanese Examined Patent Publication No. Sho 61-21505, luminance improvement was attempted by replacing a part of the composition with Mu.
Furthermore, as disclosed in, for example, Japanese Examined Patent Publication No. Hei 6-60354, luminance improvement was attempted by replacing a part of the composition with Dy or Pr.
Even these improved phosphors, however, are not enough to satisfy the requirements of recent cathode ray tubes carrying a large quantity of current to provide higher solution and luminance degradation is still a problem. Improvement to luminance is constantly required.
Luminance degradation to a great degree was regarded as a drawback of the above-mentioned conventional phosphor composition Y2SiO5:Tb.
The object of the present invention is to provide phosphors to emit high-luminance light with less luminance degradation and provide an imaging device producing good-quality images using such phosphors.
The above object can be achieved by synthesizing a phosphor that is specified below in chemical composition.
In one aspect, the invention provides a green emitting phosphor whose composition is expressed by chemical formula:
{(Y1xe2x88x92yxe2x88x92zMyGdz)1xe2x88x92xTbx}2(Si1xe2x88x92bGebO2)1+aO3
where values of x, y, z, a, and b are assigned, subject to 0 less than xxe2x89xa60.5, 0xe2x89xa6yxe2x89xa61, 0xe2x89xa6zxe2x89xa61 (where 0 less than y+zxe2x89xa61), 0 less than axe2x89xa61, and 0xe2x89xa6bxe2x89xa61, and M is at least one element (rare earth) to be selected from a group of Sc, In, La, Lu, Yb, Ce, Eu, Sm, Tm, Ho, Er, and Nd.
Green emitting phosphors synthesized by prior art are those obtained by replacing a part of component Y of parent material Y2xe2x88x922xSiO5 activated by Tbx with Gd, Sc, Yb, Eu, Sm, Tm, Mn, Dy, Pr, or the like.
In contrast, the first phosphor of the present invention is a green emitting phosphor that has composition including an excess of SiO2 in terms of stoichiometric ratio (0 less than axe2x89xa61), wherein Y is replaced with Gd, its fundamental composition being expressed by chemical formula:
{(Y1xe2x88x92zGdz)1xe2x88x92xTbx}2(SiO2)1+aO3.
The second phosphor of the present invention is a green emitting phosphor that has composition including an excess of SiO2 in terms of stoichiometric ratio (0 less than axe2x89xa61), wherein Y is replaced with at least one element (rare earth) of the following: Sc, In, La, Lu, Yb, Ce, Eu, Sm Tm, Ho, Er, and Nd, its fundamental composition being expressed by chemical formula:
{(Y1xe2x88x92yMy)1xe2x88x92xTbx}2(SiO2)1+aO3.
The third phosphor of the present invention is a green emitting phosphor that has composition including an excess of SiO2 in terms of stoichiometric ratio (0 less than axe2x89xa61), wherein Y is replaced with Gd and at least one element of the following: Sc, In, La, Lu, Yb, Ce, Eu, Sm Tm, Ho, Er, and Nd, its fundamental composition being expressed by chemical formula:
{(Y1xe2x88x92yxe2x88x92zMyGdz)1xe2x88x92xTbx}2(SiO2)1+aO3.
The fourth phosphor of the present invention is a green emitting phosphor derived from the foregoing three phosphor compositions wherein a part or all of Si is replaced with Ge, its fundamental composition being expressed by chemical formula:
{(Y1xe2x88x92yxe2x88x92zMyGdz)1xe2x88x92xTbx}2(Si1xe2x88x92bGebO2)1+aO3.
In accordance with the present invention described above, phosphors can be obtained that emit higher luminance light with less luminance degradation than those obtained by prior art.
For a phosphor obtained, according to the above-mentioned Japanese Laid-Open No. Hei 2-289679, which is an example of prior art, its result data showed that the luminance of its light emission reaches the maximum around z=0.2, where z is Gd replacement quantity. The luminance of light emission by the phosphors of present invention reaches the maximum at lower concentration of Gd (0xe2x89xa6zxe2x89xa60.1). This is because Gd replacement of less quantity is effective for generating high-luminance light in the phosphor synthesis method of the present invention.
The phosphor synthesis method according to the present invention includes a process in which reaction takes place at high temperature without using flux, which will be detailed in the later section of the preferred embodiments, and Y oxide material including Gd is used as a material of phosphor composition. This process can enhance the effect of the present invention.
For phosphors obtained by replacing a part of Si with Mn, Dy, or Pr, according to the above-mentioned Japanese Examined Patent Publication No. Sho 61-21505 and No. Hei 6-60354, which are also examples of prior art, the initial luminance of their light emission is improved, but more luminance degradation may occur as they continues to emit light. On the other hand, the properties of the phosphors obtained by the present invention are improved with regard to luminance degradation as well.
The phosphors of the present invention are obtained in unrestricted form; they may be either a single crystal or polycrystal. They may be obtained in any form such as sintered solids or powders. However, powders resulting from reaction at high temperature are often used for light emission by electron excitation in cathode ray tubes or the like. For this application, powders with a particle diameter of 1 xcexcm to 20 xcexcm are used.
In another aspect of the invention, a phosphor of the present invention is mixed with another phosphor with different composition into a phosphor mixture. Particularly, it is mixed with at least one phosphor selected from a group comprising Y3(Al, Ga)5O12:Tb, Zn2SiO4:Mn, LaOCl:Tb, and InBO3: Tb. The resultant phosphor mixture can emit higher luminance light or provide better color reproduction quality.
As concerns practical usage of phosphors obtained by the present invention, by applying a phosphor layer comprising the phosphors of the present invention to an imaging device, the invention can provide an imaging device producing good-quality images. This will be explained, using a projection-type display as an example. The projection-type display is made up of three projection tubes that are for discrete R, B, and G colors. As phosphors that are deposited to the face plate of the green projection tube, it is advisable to use phosphors of a single type of the invention or above-described green phosphor mixtures including any type of phosphors of the invention. Thereby, an imaging device featuring a longer life span can be made.
The phosphors obtained by the prevent invention are also used in cathode ray tubes for direct-viewing display (hereinafter referred to as direct-viewing tubes). As green-emitting phosphors of the phosphors of three primary colors to be applied to a face plate, it is advisable to use phosphors of a single type in accordance with the invention or above-described mixtures of any type of phosphors in accordance with the invention and another kind of green-emitting phosphors. Thereby, an imaging device featuring fine green emission color, a longer life span, shorter afterglow, and good image quality can be made.
It is also advisable to apply a phosphor layer comprising the phosphors of the present invention to an imaging device using low energy electron beams, such as a field-emitter display (abbreviated to FED). Thereby, an imaging device featuring a longer life space can be made.
The phosphors obtained by the present invention have excellent properties especially with regard to luminance degradation when being excited by large current and luminance saturation. Thus, they are most suitable for use in projection tubes and FED.
Furthermore, it is advisable to apply a phosphor layer comprising the phosphors of the present invention to an imaging device in which light emission is performed through excitation of the phosphors by ultraviolet radiation, such as a plasma display panel (PDP). Thereby, an imaging device featuring a longer life span can be made.